This invention relates to the preparation of thermally stable, substantially polycrystalline silicon carbide ceramic fibers derived from a polycarbosilane resin. The unexpected thermal stability of these fibers is achieved by the incorporation of boron prior to ceramification.
Silicon carbide ceramic fibers are well known in the art for their mechanical strength at high temperatures. As such, they have found a broad array of utilities such as reinforcement for plastic, ceramic or metal matrices to produce high performance composite materials or the formation of fibrous products such as high temperature insulation, belting, gaskets and curtains.
Several methods have been developed to manufacture such fibers. For instance, it is well known that organosilicon polymers may be spun into a fiber, infusibilized (cured) to prevent melting and ceramified at elevated temperatures. Unfortunately, this method often introduces substantial amounts of oxygen or nitrogen into the fiber through incorporation in the polymer or introduction during spinning, infusibilization or ceramification. When these fibers are heated to temperatures above 1400.degree. C., the oxygen and nitrogen is lost causing weight loss, porosity and decreased tensile strength.
Recently, polycarbosilane preceramic polymers which have a Si-C skeleton have been utilized to minimize the incorporation of nitrogen and oxygen. Yajima et al. in U.S. Pat. Nos. 4,052,430 and 4,100,233, for example, teach a method of producing silicon carbide fibers by spinning, infusibilizing and pyrolyzing various polycarbosilanes. Nippon Carbon Co., moreover, utilize this technology to produce a SiC ceramic fiber under the trade name NICALON.TM.. These fibers too, however, are known to contain about 9-15% oxygen and, thus, degrade at temperatures as low as 1200.degree. C. (see Mah et al., J. Mat. Sci. 19, 1191-1201 (1984)
The addition of other elements to polycarbosilanes has also been suggested as a means to improve the mechanical strength of SiC based bodies. For example, Yajima et al. in U.S. Pat. No. 4,359,559 disclose the production of polymetallocarbosilanes by mixing a polycarbosilane with a titanium or zirconium containing organometallic compound. Similarly, Yajima et al. in U.S. Pat. No. 4,347,347 teach the formation of an organometallic block copolymer composed of a polycarbosilane portion and a polymetallosiloxane portion wherein the metallic element is titanium or zirconium. Yajima et al. in U.S. Pat. No. 4,342,712 also teach the formation of titanium, silicon and carbon-containing ceramic fibers from a block copolymer of polycarbosilane and a titanoxane.
Yajima et al. in U.S. Pat. No. 4,248,814 teach a process for producing ceramic bodies comprising (1) preparing a polycarbosilane partly containing siloxane bonds by heating a mixture of a polysilane with 0.01 to 15 weight percent polyborosiloxane, (2) mixing the resultant polycarbosilane with a ceramic powder and (3) sintering at a temperature of from 800.degree. C. to 2000.degree. C. This process, however, fails to teach the formation of ceramic fibers.
Yajima et al. in U.S. Pat. Nos. 4,220,600 and 4,283,376 teach the formation of a polycarbosilane partly containing siloxane bonds by a process comprising adding 0.01 to 15 weight percent polyborosiloxane to a polysilane and then heating. This polycarbosilane can then be spun, cured and pyrolyzed to form silicon carbide ceramic fibers. Pyrolysis temperatures up to 1800.degree. C. are disclosed in the reference but the examples only teach pyrolysis up to 1300.degree. C.
The incorporation of these elements, however, is often accompanied by various problems. For instance, high temperature and pressure is often necessary to cause the incorporation. The yields of the resulting polymers are often low. Additionally, the elements often bond to the silicon though intermediate oxygen linkages so that increasing amounts of oxygen are present in the polymer. Further, silicon carbide based fibers so produced are typically composed of extremely fine crystalline grains; heating the fibers to temperatures of 1300.degree. C. or higher causes growth of the grains which results in a decrease in mechanical strength of the fibers.
Takamizawa et al. in U.S. Pat. No. 4,604,367 teach the preparation of an organoborosilicon polymer by mixing an organopolysilane with an organoborazine compound, spinning fibers and then ceramifying the fibers by heating to temperatures in the range of 900.degree.-1800.degree. C. However, the actual examples in this reference only show heating up to 1300.degree. C. and the tensile strength of the fibers is reported to drop off dramatically when heated above 1500.degree. C. (note the graph on the cover of the reference)
Takamizawa et al. in U.S. Pat. No. 4,657,991 teach the formation of SiC fibers by using a composition comprising a polycarbosilane and a silmethylene polymer which may be copolymerized with an organometallic compound containing boron, aluminum, titanium or zirconium. After spinning the above composition, the fibers are pyrolyzed to between 800.degree. and 1500.degree. C. The inventors therein teach that pyrolysis at temperatures above 1500.degree. C. decreases the mechanical strength of the resulting fiber due to grain size growth.
The present inventors have now unexpectedly found that thermally stable, substantially polycrystalline SiC fibers can be formed from polycarbosilanes with greater than about 0.2% boron incorporated therein and firing said fibers to greater than about 1600.degree. C.